1. Field of the Invention
This invention relates to arrangements of plural fuel cell stacks, and more particularly to ducting arrangements for stacks operating with separate fuel, oxidant and coolant streams.
2. Description of the Prior Art
Among the various types of fuel cell systems are those which include subassemblies of two bipolar plates between which is supported an acid electrolyte in a matrix. The subassemblies, herein referred to as fuel cells, are oriented one atop another and electrically connected, typically in series, to form a fuel cell stack. Operation of the fuel cell, for example, the reaction of hydrogen and oxygen to produce heat, electrical energy and water, is exothermic, and cooling of the cell components is necessary in order to maintain component integrity. Liquid or gaseous cooling fluids have therefore been passed through the stack to remove heat. Accordingly, three fluid mediums, a fuel, an oxidant and a cooling fluid flow in some manner into and out of the fuel cell stack.
As a result of system design limitations, such as the geometric configuration and the need to provide sufficient cooling of the cells, fluid streams have typically been combined or require relatively complex sealed ducting configurations to direct flow of the various fluid mediums. For example, systems have been proposed wherein the cooling fluid and oxidant are the same medium, such as air. Such systems require a high circulatory power, detracting from overall system efficiency, and can subject downstream components, such as heat exchangers, to undesirable materials such as corrosive acid, carried with the depleted oxidant. Further, in combined systems providing energy generation utilizing the heat removed from the fuel cell stack, it is desirable to recover the heat at a high temperature, which is limited by excessive air flow rates. Other systems, such as that described in U.S. Pat. No. 4,074,020, require flow paths which enter a longitudinal end of the stack, flow radially through selected sections of the stack, and are discharged longitudinally, creating multiple flow paths and large pressure drops. Multiple, spaced inlets and outlets also require complex ducting arrangements.
Control of the fluid mediums into and out of the fuel cell stacks is more complex where multiple stacks are utilized. Separate ducting to each stack, or to each discrete level in a stack, can result in a maze of interconnections. The connections can be simplified where the coolant and oxidant are the same medium. Proposed, for example, have been four rectangular stacks arranged generally in a square array with shorter ends near one another. Fuel flows from one shorter face of the rectangle to another, and the combined oxidant and coolant flows from one longer face of the rectangle to the other, providing a 90.degree. cross flow of coolant and oxidant relative to the fuel. The assembly forms an inert duct-like central channel bounded by the four stacks. A single duct at each of two corners of the square array feeds fuel to two consecutive stacks, and a single duct at each of the remaining two corners withdraws fuel from two consecutive stacks. Combined coolant and oxidant flows from outside the square array, through the stacks, and into the central channel. The utilization of a combined coolant and oxidant flow stream severly limits the choice of coolant and the cooling capacity for the stacks.